1. Field of the Invention
The present invention relates to the deployment of Video on Digital Subscriber Lines (DSL) and specifically to significantly improve the protection of DSL systems against a wide variety of impulse noises experienced in the field, in order to maintain a high QoS and an acceptable user experience, even in a non-stationary environment.
2. Related Art
High-bandwidth systems, including DSL systems, use single-carrier modulation as well as multi-carrier modulation schemes. Both DSL and other high-bandwidth systems such as wireless use modulation schemes such as Carrier-less Amplitude and Phase Modulation (CAP) and Discrete Multi-tone (DMT) for wired media and Orthogonal Frequency Division Multiplexing (OFDM) for wireless communication. One advantage of such schemes is that they are suited for high-bandwidth application of 2 Mbps or higher upstream (subscriber to provider) and 8 Mbps or higher downstream (provider to subscriber). Quadrature Amplitude Modulation (QAM) utilizes quadrature keying to encode more information on the same frequency by employing waves in the same frequency shifted by 90°, which can be thought of as sine and cosine waves of the same frequency. Since the sine and cosine waves are orthogonal, data can be encoded in the amplitudes of the sine and cosine waves. Therefore, twice as many bits can be sent over a single frequency using the quadrature keying. QAM modulation has been used in voice-band modem specifications, including the V.34.
CAP is similar to QAM. For transmission in each direction, CAP systems use two carriers of identical frequency above the 4 kHz voice band, one shifted 90° relative to the other. CAP also uses a constellation to encode bits at the transmitter and to decode bits at the receiver. A constellation encoder maps a bit pattern of a known length to a sinusoid wave of a specified magnitude and phase. Conceptually, a sinusoidal wave can be viewed to be in one-to-one correspondence with a complex number where the phase of the sinusoidal is the argument (angle) of the complex number, and the magnitude of the sinusoidal wave is the magnitude of the complex number, which in turn can be represented as a point on a real-imaginary plane. Points on the real-imaginary plane can have bit patterns associated with them, and this is referred to as a constellation and is known to one of ordinary skill in the art.
DMT modulation, sometimes called OFDM, builds on some of the ideas of QAM but, unlike QAM, it uses more than one constellation encoder where each encoder receives a set of bits that are encoded and outputs sinusoid waves of varying magnitudes and phases. However, different frequencies are used for each constellation encoder. The outputs from these different encoders are summed together and sent over a single channel for each direction of transmission. For example, common DMT systems divide the spectrum from 0 kHz to 1104 kHz into 256 narrow channels called tones (sometimes referred to as bins, DMT tones or sub-channels). These tones are 4.3125 kHz wide. The waveforms in each tone are completely separable from one another. In order to maintain separability, the frequencies of the sinusoidal used in each tone should be multiples of a common frequency known as the fundamental frequency and in addition the symbol period τ, must be a multiple of the period of the fundamental frequency or a multiple thereof. The aggregate bit pattern which comprises the bit patterns mapped to constellations in each of the tones during a symbol period is often referred to as a DMT symbol. For the purposes here, time is often referred to in terms of DMT symbols meaning a symbol period.
The presence of impulse noise can occur in digital subscriber line (XDSL) systems due to electromagnetic interference from such sources as a telephone network, power system, and even from natural phenomena such as thunderstorms and lightning. The presence of impulse noise can significantly limit the reliability of real-time services such as video that can be supported by current generation xDSL systems, e.g., VDSL (Very High Speed DSL). In particular, impulse noise can cause physical layer cyclic redundancy check (CRC) errors and loss of packets in xDSL systems, thereby affecting such triple-play services as IPTV. Therefore, there has been substantial interest in the DSL community recently towards the development and standardization of impulse noise protection schemes.
Typically, the most common forms of impulse noise that are observed on a line are repetitive electrical impulse noise (REIN), and a short high impulse noise event (SHINE). Electromagnetic interference from telephone networks, fluorescent lights, power supply units of TVs or PCs, video recorders, electronic transformers, etc. in the vicinity of a particular line, can cause noise impulses having a repetitive character often proportional to the frequency of the power lines (60 Hz in the U.S. and 50 Hz in Europe). This is an example of REIN. Lightning would be an example of a source of SHINE.
To better understand approaches to combating impulse noise, the particular transmission layers involved in DSL are explained. FIG. 1 illustrates DSL communications layering. For the sake of example here, transmission is described from a transmitter (TX) 152 to a receiver (RX) 154 where the transmitter can be a central office (CO) and the receiver, a customer premises equipment (CPE). However, analogously the transmitter can be a CPE and the receiver can be a CO.
Within (TX) 152 is layer 122, the Transport Protocol Specific-Transmission Convergence (TPS-TC) layer. At this layer is the transport; application specific transports are implemented such as ATM or Ethernet. This layer can be further subdivided into a network processor layer which lies above a gamma interface.
The next layer is layer 124, the Physical Media Specific Transmission Convergence (PMS-TC) layer. This layer manages framing, transmission, and error control over the line. In particular, this layer comprises the forward error correction (FEC) codes such as the Reed-Solomon (RS) Codes. The interface between the TPS-TC and the PMS-TC layer is referred to as the alpha layer. It is very common that PMS-TC layer 124 comprises scrambler-RS encoder 102 and interleaver 104 to implement a RS code with interleaving (RS-ILV). DSL standards mandate the use of RS as the FEC.
The next layer is layer 126, the physical media dependent (PMD) layer also referred to as the physical (PHY) layer. This layer encodes, modulates and transmits data across physical links on the network. It also defines the network's physical signaling characteristics. In particular it translates data into symbols through the use of inverse fast Fourier Transforms and Trellis codes into DMT symbols. A Trellis code can also supply additional error correction. The interface between the PMD layer and the PMS-TC layer is referred to as the delta interface.
After processing by the PMD layer, the data is transmitted across DSL loop 140, where it is received by RX 154 using its PMD layer 136. In a receiving capacity PMD layer 136 decodes, demodulates and receives data across physical links on the network. Furthermore, PMS-TC layer 134 in a receiving capacity decodes data encoded by PMS-TC 124 and extracts data from the framing scheme. In particular it can comprise de-interleaver 106 and descrambler-RS decoder 108 to extract data encoded by PMS-TC 124. Finally, layer 132 is the receiving counterpart of the TPS-TC layer.
As mentioned above, in a legacy DSL system, RS-ILV is used as an FEC code. One difficulty with this approach is that the amount of redundancy built in the RS-ILV coding scheme shall be proportional to the amount of data corrupted by the impulse. Ultimately, the maximum allowed amount of redundancy may be insufficient to correct errors caused by long duration impulse noise such as SHINE. Furthermore, the more redundancy built into the RS-ILV code the lower the throughput. Increasing the redundancy in order to accommodate rare long duration impulse noise events is wasteful of bandwidth.
FIG. 2 illustrates a more detail description of the transmission side of the PMS-TC layer as disclosed by present xDSL standards. Data from the TPS-TC layer transmitted to the PMS-TC layer can use one of two latency paths and one of two bearer channels. Input 202 represents data on the first bearer channel designated for latency path #0. Input 204 represents data on the second and optional bearer channel designated for latency path #0. Input 206 represents data on the first bearer channel optionally designated for latency path #1. Input 208 represents data on the second bearer channel optionally designated for latency path #1. In addition, overhead data can be received by the PMS-TC layer including Embedded Operations Channel (EOC) 210, Indicator Bits (IB) Channel 212 and Network Timing Reference (NTR) 214, which can be combined by multiplexer (MUX) 218. In addition, MUX 216 combines inputs 202 and 204, and MUX 220 as part of optional latency path #1 combines input 206 and 208. MUX 222 combines the output of MUX 216 with a sync byte and overhead data from MUX 218. Similarly MUX 224 combines the output of MUX 220 with a sync byte and overhead data. Along each latency path, scrambler 222 and corresponding scrambler 224 scramble the data received from MUX 222 and MUX 224, respectively. FEC 226 and FEC 228 apply an FEC to the scrambled data from scramblers 222 and 224, respectively. Typically, the FEC used is a RS code and in the example of FIG. 1, scrambler-RS 102 comprises scramblers 222 and 224 and FECs 222 and 224, but are shown consolidated for compactness in FIG. 1. Interleaver 104 of FIG. 1 comprises interleaver 230 and interleaver 232 which performs the interleaving of the encoded data received from FEC 226 and FEC 228, respectively. Finally, MUX 234 combines the encoded interleaved data for both latency paths to produce output 240 which is ready for processing by the PMD layer.
It should be noted that the architecture of FIG. 3 is applicable to both transmission from the CO to the CPE (downlink) and from the CPE to the CO (uplink).
One approach taken in the past is to retransmit at the alpha interface. FIG. 3 illustrates a DSL system using retransmission. CO 352 is like (TX) 152 but further comprises a retransmission module 302. Whenever data is supplied from TPS-TC to PMS-TC, the data is framed into data transmission units (DTUs), then copied and stored in a retransmission module 302, e.g., in a first-in-first-out (FIFO) memory. Typically a DTU is one or more RS code words, but can be tied to other abstractions from the TPS-TC layer such as a block of ATM cells, or PTM cells. Similarly, CPE 354 like CO 352 comprises retransmission module 302.
When retransmission control module 304 detects a corrupt DTU such as using the failure indication of the RS decoder, a request for retransmission is sent from CPE 354 to CO 352 and more specifically from retransmission control module 302 to retransmission module 302. In order to identify the DTU to be retransmitted, each DTU must have a sequence identifier (SID) added. FIG. 4 shows the transmission side of a PMS-TC and retransmission module. The block diagram is similar to that which is standards based as shown in FIG. 2. Retransmission module 302 is shown inserted into the input path of latency path #0. Prior to retransmission module 302, SID 402 and retransmission control channel (RCC) 404 are combined into each DTU by MUX 304. On the CO side, SID 402 is used to identify the DTU. However, when CPE 254 requests retransmission, a request is inserted into the uplink transmission through RCC 404 along with the SID of the corrupt DTU. To clarify, in the context of downlink transmission, SID 402 is used to identify a DTU. In the context of retransmission, SID 402 and RCC 404 are combined into the uplink transmission. Upon receiving the request by retransmission module 202, the CO 352 retransmits the corrupted DTU.
The prior retransmission solutions offer a number of alternatives for incorporating the SID in the downlink transmission and the RCC in the uplink transmission. FIG. 5 illustrates an alternative where retransmission module 504 resides within the PMS-TC layer. FIG. 6 illustrates another alternative where retransmission module 1004 resides just above the delta layer.
One difficulty with this approach is that the retransmission time because of the need to traverse the PMS-TC layers on both the CO and CPE can be as high as 5 milliseconds. Furthermore, a retransmission approach can clog up transmission over the DSL loop if a REIN source exhibits a short inter-arrival-time (IAT). For example, if the IAT of a REIN source is less than the round trip time of the retransmission request, a new impulse would be experienced causing another retransmission.
Another difficulty is that in both the transmission of the DTU and the retransmission request a SID needs to be transmitted. This can particularly be a drain on bandwidth in an ADSL system where the uplink capacity is smaller than the downlink. Additionally, the use of retransmission is essentially mutually exclusive to the FEC rather than cooperative. The FEC only serves as a detector for retransmission and its error correcting capabilities are not fully exploited.
Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.